A so-called Co-doped type of ferromagnetic iron oxide is used as ferromagnetic particles having a high coercive force (Hc) which is necessary for high density magnetic recording. The Co-doped type iron oxide can be prepared, for example, by adsorbing amorphous cobalt compounds such as cobalt hydroxide (Co(OH).sub.2) on acicular iron oxyhydroxide (acicular .alpha.-FeOOH) (referred to as "acicular goethite" hereinafter), washing with water, filtering, drying, and then calcining (dehydrating, reducing, and, if necessary oxidizing) to prepare acicular iron oxide particles (.gamma.-Fe.sub.2 O.sub.3), and simultaneously diffusing cobalt ion into the thus prepared acicular iron oxide particles. Hereinafter the above method is called a "Co-doping method".
The process of the Co-doping method is relatively simple and the manufacturing cost is relatively low, but there are some defects in that the thus obtained magnetic particles are easily influenced by temperatures, for example, the Hc (coercive force) changes about 100 Oe with a 10.degree. C. difference in temperature, demagnetization heavily occurs by adding pressure and by heating and print through phenomenon easily occurs in a magnetic recording medium prepared by using the above-described magnetic particles.
In order to remove these defects, it has recently been proposed, for example, as disclosed in Japanese Patent Publication Nos. 49475/74 and 29157/75, and Japanese Patent Application (OPI) Nos. 74399/74 and 37667/75 (the term "OPI" as used herein means an "unexamined published application"), that instead of diffusing cobalt into .gamma.-Fe.sub.2 O.sub.3 particles, .gamma.-Fe.sub.2 O.sub.3 particles be coated with cobalt, or modified by cobalt to prepare ferromagnetic iron oxide.
Those magnetic particles are prepared by water-washing, filtering, drying and calcining (dehydrating, reducing, and, if necessary oxidizing) an aqueous suspension of, for example, acicular goethite particles to prepare acicular magnetic iron oxide (.gamma.-Fe.sub.2 O.sub.3), and then adding cobalt hydroxide, cobalt sulfate, ferrous hydroxide, an alkali and the like to the aqueous suspension to prepare the product by a so-called wet reaction. Hereinafter this method is called a "Co-modifying method".
The Co-modified ferromagnetic iron oxide particles prepared by the above Co-modifying method have favorable characteristics in that these iron oxide particles are hardly influenced by temperature differences (i.e., changing several Oe with 10.degree. C. difference), demagnetization caused by adding pressure and by heating is low and print through phenomenol hardly occurs. However, the manufacturing process is complicated, the manufacturing period is long, and the manufacturing cost also becomes high, because as is clear from the above-described process, once acicular magnetic iron oxide (.gamma.-Fe.sub.2 O.sub.3) is formed, Co modification must then be conducted by wet reaction.